Field of the Invention
The present invention relates to a collision type pneumatic pulverizer for pulverizing powder material using a jet of gas (a high-pressure gas), and a process for producing a toner used for developing electrostatic images using such a pulverizer.
Related Background Art
Collision type pneumatic pulverizers which employ a jet of gas are designed to pulverize powder material by ejecting a flow of particle-gas mixture obtained by mixing a jet of gas with powder material from the outlet of an accelerating tube and thereby making the flow of particle-gas mixture collide against the colliding surface of a colliding member provided in front of the outlet of the accelerating tube.
A conventional collision type pneumatic pulverizer will be described in detail below with reference to FIG. 8.
In a conventional collision type pneumatic pulverizer, a colliding member 4 is provided in opposed relation to an outlet 13 of an accelerating tube 3 connected to a high-pressure gas supply nozzle 2. The powder material is sucked into the accelerating tube 3 from a powder material supply port 1 connected to the intermediate portion of the accelerating tube 3 by the flow of a high-pressure gas supplied to the accelerating tube 3. The sucked powder material is ejected together with the high-pressure gas in such a manner that it collides against the collision surface of the colliding member 4. Consequently, the powder material is pulverized due to the impact of the collision.
In the above-mentioned type of conventional pulverizer, the colliding surface 14 of the colliding member has a flat surface which is perpendicular to or inclined (by, for example, 45.degree.) with respect to the direction of the flow of powder material-gas mixture (the axial direction of the accelerating tube), as shown in FIG. 8 or 9. The pulverizer having the former type colliding member is disclosed in Japanese Patent Application Laid-Open No. 57-50554, and the pulverizer having the latter type colliding member has been proposed in Japanese Patent Application Laid-Open No. 58-143853).
In the pulverizer shown in FIG. 8, the powder material having a large particle size is supplied into the accelerating tube 3 from the supply port 1. The supplied powder material is made to collide against the colliding surface 14 of the colliding member 4 by the jet of gas supplied from the jet nozzle 2 and is thereby pulverized. The pulverized powder is discharged to the outside of a pulverizing chamber from an exhaust port 5. However, in a case where the colliding surface 14 is perpendicular to the axial direction of the accelerating tube 3, the rate at which the powder material ejected from the jet nozzle 2 and the powder reflected by the colliding surface 14 co-exist near the colliding surface 14 is high, and the concentration of the powder near the colliding surface 14 is thus high, deteriorating the pulverization efficiency. Furthermore, the main collision is the primary collision on the colliding surface 14, and the secondary collision on a wall 6 of the pulverizing chamber is not effectively utilized. Furthermore, in a pulverizer in which the colliding surface is perpendicular to the accelerating tube 3, pulverization of a thermoplastic resin readily generate melting or agglomeration of the powder material due to the local heating generated during collision. Therefore, stable operation of the pulverizer is difficult and the pulverization capability is reduced. These in turn make the use of the powder at a high concentration difficult.
In the pulverizer shown in FIG. 9, since the colliding surface 14 is inclined with respect to the axial direction of the accelerating tube 3, the concentration of the powder near the colliding surface 14 can be reduced as compared with that in the pulverizer shown in FIG. 8. However, the pulverization pressure is dispersed and hence reduced. Furthermore, the secondary collision provided by the wall 6 of the pulverizing chamber cannot be utilized effectively.
In a pulverizer such as that shown in FIG. 9 in which the colliding surface 14 is inclined by 45.degree. with respect to the accelerating tube, occurrence of the aforementioned problems can be lessened during pulverization of a thermoplastic resin. However, the impact force used in pulverization during collision is low, and pulverization by the secondary collision on the wall 6 of the pulverizing chamber is reduced. Thus, the pulverization capability is reduced to 1/2 or 1/1.5 of that of the pulverizer shown in FIG. 8.
Japanese Utility Model Application Laid-Open No. 1-148740 and Japanese Patent Application Laid-Open No. 1-254266 disclose the collision type pneumatic pulverizers which can overcome the aforementioned problems.
In the pulverizer disclosed in Japanese Utility Model Application Laid-Open No. 1-148740, a colliding surface 15 of a colliding member is disposed at a right angle with respect to the axis of the accelerating tube, and a conical protrusion 14 is provided on the colliding surface 15 so as to prevent generation of a reflected flow by the colliding surface, as shown in FIG. 11.
In the collision type pneumatic pulverizer proposed in Japanese Patent Application Laid-Open No. 1-254266, the distal end portion of the colliding surface of the colliding member is made conical, as shown in FIG. 10, so as to reduce the concentration of the powder near the colliding surface and thereby provide effective secondary collision on the wall 6 of the pulverizing chamber. The above-mentioned structures can eliminate the problems of the conventional pulverizers to a certain degree but cannot solve them sufficiently. Also, in recent years, there have been a demand for finer pulverization and for a pulverizer exhibiting better pulverization efficiency.
The toner or the coloring resin powder for toner used in the image forming method by the electrophotographic process generally contains at least a binder resin and a coloring agent or magnetic powder. The electrostatic image formed on a latent image carrier is developed by the toner, and the developed toner image is transferred onto a transfer material, such as normal paper or a plastic film. The toner image on the transfer material is fixed by a fixing device, such as a heating fixing means, a pressure roller fixing means or a heating and pressurizing roller fixing means. Thus, the binder resin used in the toner is characterized in that it is plastically deformed when applied with heat and/or pressure.
A currently employed method of preparing the toner or the coloring resin powder for toner includes the steps of melting and kneading a mixture consisting of at least a binder resin and a coloring agent or magnetic powder (and a tertiary component, when necessary ), cooling the kneaded mixture, pulverizing the cooled mixture and classifying the pulverized material. Pulverization of the cooled material generally consists of coarse grinding (or intermediate grinding) by a mechanical impact type pulverizer, and subsequent fine pulverization by a collision type pneumatic pulverizer using a jet of gas.
In the collision type pneumatic pulverizer using a jet of gas, the powder material is conveyed by a jet of gas and is made to collide against the colliding member, whereby the powder material is pulverized.
The conventional pulverizers are those shown in FIGS. 8, 9, 10 and 11.
The toners for developing the electrostatic image manufactured using the collision type pneumatic pulverizers shown in FIGS. 8, 9, 10 and 11 are substantially satisfactory but could be improved. Also, in recent years, there has been a demand for a toner having a smaller particle size which can be used to achieve high definition and high quality images and a demand for a method of manufacturing a toner more efficiently.